A belt-drive continuously variable transmission for use in a vehicle comprises a primary pulley which inputs engine torque, a secondary pulley which is connected to the drive wheel, and a V-belt which is wrapped around the pulleys. The V-belt engages with grooves formed respectively in the primary pulley and secondary pulley. The width of the grooves varies according to the oil pressure that is applied to the primary pulley and secondary pulley, and the radius of contact between the V-belt and each pulley varies accordingly. By means of these variations in the contact radius, the speed ratio of the pulleys changes continuously.
Control of the speed ratio is performed by a controller which is constituted by a microcomputer. The controller sets a target speed ratio on the basis of vehicle operating conditions such as the engine rotation speed and throttle opening. Meanwhile, the actual speed ratio is calculated from the rotation speed of the primary pulley and secondary pulley. The amount of oil pressure supplied to the primary pulley and secondary pulley is then increased or decreased such that the actual speed ratio matches the target speed ratio.
In such a belt-drive continuously variable transmission, if for some reason the oil pressure falls temporarily, the holding force of the pulleys in regard to the belt decreases, possibly causing the V-belt to slip along the pulleys.
Tokkai Hei 11-139184, published by the Japan Patent Office in 1999, proposes that in such a case, the engine output torque be reduced in order to decrease the input torque of the transmission.